To suspend a VM on host shutdown you will need to configure the following file

vi /etc/sysconfig/libvirt-guests

Set the following options (which are located throughout the configuration file).

ON_BOOT=start
START_DELAY=5
ON_SHUTDOWN=suspend

You might not want to use START_DELAY=5, but I would rather the virtual machines start up with a little staggering rather than concurrently.

The libvirt-guests service also needs to be enabled and started.

systemctl start libvirt-guests
systemctl enable libvirt-guests

You will now be able to reboot the host machine to install updates etc. and the virtual machines will be suspended and restored rather than shutdown interrupting their jobs.